Ignition coils of this type are conventionally configured to have an extremely minimized volume and weight, and they are predominantly used in internal-combustion engines in which each combustion cylinder is equipped with its own ignition coil and rests directly on the spark plug without expensive mounting elements. Ignition coils of this type are also known as single-spark ignition coils or rod ignition coils and have to be particularly vibration-resistant and able to withstand high temperatures, as they make direct contact with the heated engine block, which generates vibrations.
In addition to the primary and secondary induction coil, a single-spark ignition coil of this type comprises a specific magnetic circuit and may also include an electronic circuit element, for example an output stage which is connected to the induction coils to form a unit. Two plug connectors, one for connection of the high-voltage terminal to the spark plug and one plug connector that generally has four pins for the power supply from the wiring and the activation line complete an ignition coil of this type. The ignition systems are activated by the engine electronics, which determine the moment of ignition from a plurality of dynamic engine characteristics.
Single-spark ignition systems of this type have advantages over an ignition system that is powered by a single ignition coil and operates by the distributor principle. High-voltage lines, including the mechanical drive and distributor assembly, which is adversely affected by wear and contamination during operation and which influence the moment of ignition or impair the ignition power, may be dispensed with.
The physical operating principles of energy transmission, described hereinafter, apply to ignition coils of this type. An externally powered primary coil and the associated build-up of a magnetic field, which leads to an inductive transfer to the secondary winding when the primary current is interrupted, are used as a starting point.
The secondary current in the high-voltage portion is built up merely by the induction principle from the reduction in magnetic flux brought about by the disconnection of the primary current and the associated change in magnetic flux. However, this build-up of current and the incipient discharge do not take place continuously, but in four phases, according to the physical parameters that are predominant in each case. The build-up of current due to the capacitance of the secondary winding begins before the actual discharge via the spark plug electrodes directly after initiation of the reduction in primary current.
The first phase of the build-up of secondary current begins without delay when the reduction in the primary current commences. The charge is shifted according to the capacitance of the secondary winding with associated formation of corresponding electric fields on the spark plug electrodes, which then bring about the actual power breakdown. A considerable reduction in primary current, starting from the maximum value of the primary current, is required for generating the electric fields necessary for the secondary power breakdown. It is approximately 30% with a duration of action of 2 to 5 μsec and is determined by the ignition coil concept and the electronic switch, which influences the speed of disconnection of the primary current.
The equation
            U      indu        =                  N        ·                  ΔΦ                      Δ            ⁢                                                  ⁢            t                              ⁢                          ⁢      or                  U      indu        =          L      ·                                    Δ            ⁢                                                  ⁢            l                                Δ            ⁢                                                  ⁢            t                          .            applies to induction and self-induction processes with loss-free observation and for an open circuit.
The second phase of the secondary current is a sudden increase of a resistive nature associated with the power breakdown. It has substantially no inductive cause and results from the capacitive discharge of the secondary winding charge that has accumulated in the first phase.
Pure induction processes are predominant in the third phase of the secondary increase in current, the change in magnetic flux acting as a difference in the associated ampere turns (primary side decreases, secondary side increases) due to the further reduction in the primary current and the resultant increase in the secondary current, and the increase in the secondary current is consequently flatter, even though the reduction in the primary current remains uniform at this moment. It flattens only toward the end of the reduction in primary current, and the increase in the secondary current attains its maximum value in a soft run-out. The physical principle of the increase in the secondary current is manifested in that, in each phase of the increase, the maximum number of secondary ampere turns that can ever be adjusted is the same as the number of ampere turns that have previously been induced on the primary side, because only the magnetic field (originally produced by the primary winding) occurs as the energy parameter during induction and, according to the principle of energy conservation, cannot propagate itself, even with such a rapid reduction in primary current.
Therefore, the following relationship applies to the phase of increase in the secondary current, with a reduction in the primary current and loss-free observation:Nsecondary×dlsecondary≦Nprimary×dlprimary 
This physical principle acts substantially independently of the speed of the primary switching operations, providing that sufficient voltage is induced to overcome the ohmic resistance. These procedures also take place independently of the presence of an iron circuit.
The fourth phase of the secondary current curve represents the magnetic free-run of the iron circuit, in particular of the magnetic coil core, the counter-induction of the secondary coil being predominate for the period of action of the magnetic free-run. The primary winding is already at zero current in this phase, and an influence on the secondary side, if significant on account of the smallness, would only be possible via capacitance.
All of the formerly known compact ignition coils, of the type shown, for example, in DE 199 62 279 A1, DE 199 27 820 C1, WO 99/36693, DE 199 50 566 A1, EP 1 111 630 A2 or EP 0 959 481 A2, run the risk of overheating during operation. This is due to the self-heating, predominantly by the considerable current load (15 amperes) of the primary winding, but also due to the power dissipation of the secondary winding. To this is added the exposure to heat due to the relatively high ambient temperature of the engine block (up to 125° C.). The lower thermal value of compact ignition coils further complicates the attainment of a state of equilibrium at a justifiably controlled temperature (maximum 160° C.), in particular during continuous operation with maximum ignition frequency.
European patent application EP 0 959 481 A2 discloses an embodiment of a compact rod ignition coil, in which the risk of overheating, in particular of the electronic output stage, is to be reduced, so that reliable operation is achieved even when it is exposed to high temperatures. Overheating is prevented passively by attempting to isolate the individual sources of heat by means of a separating gap. However, this solution has the drawback that the actual production of undesirable heat is not counteracted.
It is accordingly an object of the present invention to provide an improved ignition coil for ignition system, in particular a rod ignition coil for internal-combustion engines, which ensures increased reliability in operation and energy efficiency as well as a lower risk of overheating during operation.